Added some pretrial spikes to NLX_DivideSpikeArray

It now stores spikes 100ms before the start of the trial. There is no
check to make sure that they do not belong to the previous trial so use
with care.
Added evets to NLX_event2num

I missed a few events when I made the function. Most important
'NLX_DIMMING1' that is very important for grcjdru1